As antennas employed in wireless communication apparatuses such as portable telephones, different antennas having different configurations have been proposed. See, for example, Japanese Unexamined Patent Application Publication No. 2003-78332 (“Patent Document 1”) and Japanese Unexamined Utility Model Application Publication No. 6-34309 (“Patent Document 2). For example, an antenna of the invention disclosed in Patent Document 1 includes a first resin which is not easily subjected to metal plating and a second resin which is easily subjected to metal plating. The antenna is formed by a two-step injection molding method so that at least part of the second resin is exposed. A conductive metal layer is plated on the second resin and a plated portion is configured as an element.
In recent years, especially there has been a demand for miniaturization of wireless communication apparatuses such as portable mobile terminals (portable telephones, for example) having wireless communication functions. Therefore, there has been a demand for miniaturization of antennas included in these portable mobile terminals. However, if antennas are miniaturized for this demand, according to the invention disclosed in Patent Document 1, there arises a problem in that radiation efficiency is deteriorated. This is because, in the invention disclosed in Patent Document 1, an element is formed on a resin by plating and the resin closely adheres to entire surfaces of a power-feeding element and a non-power-feeding element. Therefore, if the antenna is to be miniaturized, a resin having high permittivity is inserted between a radiation electrode and the ground. As a result, it is difficult to emit an electric field to the outside, and therefore, the radiation efficiency is deteriorated.
Furthermore, in the antenna according to the invention disclosed in Patent Document 1, for example, a line width and a length of a current path are adjusted in order to set a resonant frequency used for antenna operation to a desired frequency. Accordingly, if the antenna according to the invention disclosed in Patent Document 1 is miniaturized, a region in which the current path is to be formed is reduced, and therefore, an efficient line length is not ensured. Accordingly, the line width of the current path becomes small. In this case, there arises a problem in that conductive loss is increased due to concentrated current, and antenna efficiency is deteriorated.